Numerical Simulation Of Gas-film Reactor For Supercritical Water Oxidation

Supercritical water oxidation is a new technology which is mainly used for the treatment of recalcitrant organic compounds. Transpiring wall reactor is the most effective solution to solve both the corrosion and salt precipitation occurred in the supercritical water oxidation process. However, according to previous research, pure water which is preheated to at least350℃is needed in the transpiring wall reactor to form water-film. In order to form subcritical area dissolving inorganic salt at the bottom of the reactor, the temperature of pure water passed into the reactor must be low, resulting in the low temperature of outflow and limiting the energy recovery.Based on this, a gas-film reactor for supercritical water oxidation is designed, in which the protect gas is N2.A11the inlet flow (fuel, N2and O2) except auxiliary heat source (H2O) are passed into the reactor at the room temperature. The low specific heat and thermal conductivity of N2can lower the temperature of porous wall and upper the temperature of outflow without influencing the reaction inner the reactor, which increase the economy effectively.A computational fluid dynamic model of a gas-film reactor for supercritical water oxidation was developed using CFD simulation software FLUENT, in which some assumption and simplification was made. Reasonable turbulence model and finite rate model were chosen during the simulation. The simulated data shows good agreement with the experimental data according to the distribution of temperature field, flow field and species field.Protective effect of gas-film to the reactor was verified by analyzing the temperature field, flow field and the tracks of inorganic particles simulated by the DPM model. The results prove that the porous wall is only under high temperature and the shell is only under high pressure. The subcritical area near porous wall and the existence of N2can prevent inorganic particles from adhering to the porous wall. Therefore the gas-film can protect the reactor very well.Effect of the operating parameters on the temperature profiles were investigated for the better design of the reactor. According to the results, the flow of auxiliary heat source has a jet entrainment effect which can enhance the mixture of components and the transfer of energy. And the jet entrainment effect is strengthened by the increase of auxiliary heat source flux. The highest temperature inside the reactor increases with the increase of flux and concentration of fuel (CH3OH), temperature of auxiliary heat source (H2O) and the ratio of fuel flux to auxiliary heat source flux and the decrease of auxiliary heat source flux, but has little to do with the N2flow. The axis temperature of reactor increases with the increase of fuel flux and concentration, auxiliary heat source temperature and the ratio of fuel flux to auxiliary heat source flux. And the varied range is influenced by the total flux greatly. But with the increase of N2flux, the axis temperature decreases obviously. The temperature of porous wall increases with the increase of fuel flux and concentration, auxiliary heat source flux and temperature, and the ratio of fuel flux to auxiliary heat source flux and decreases with the increase of N2flux. With the increase of auxiliary heat source flux and the decrease of the ratio of fuel flux to auxiliary heat source flux, the region of high temperature moves upper.A new method using the heat load parameters as criterion for the design of the gas-film reactor was proposed. The temperature of the whole reactor decreases when the diameter of the reactor increases, and the high temperature region moves down. The reactor’s length can only increases the residence time and decreases the temperature of outflow. The heat load data are obtained at certain criterions:the useful reaction time must be larger than11s which is the time fuel needs to degrade completely; the temperature of porous wall must be lower than374℃to form subcritical zone. The relationship with the critical fuel flux, the diameter of the reactor and the N2flux can be expressed as lnFf=1.3x10-5d+0.001578FN2+3.4, and the relationship between the critical fuel flux and the reactor size can be expressed by the formation:Ff=(0.4265d-0.01188)L-1.04×10-4+3.3.